While trying to develop epoxy resins with a low coefficient of thermal expansion for composites, scientists at Washington State University (WSU) and Oak Ridge National Laboratory went a step or two beyond their original goal. The result is an epoxy resin liquid crystal network (LCN) with azobenzene chromophores and reversible ester bonds that displays three-way shape memory behavior that can be triggered by heat or light. Three-way shape memory behavior involves automatic transitions between three shapes of a material—a free-stress shape and two temporary, intermediate shapes. To top it off, this thermosetting resin can also heal itself when damaged.

In this three-component system, the epoxy LCN provides the shape memory functionality, the azobenzene chromophores contribute photomechanical properties, and the dynamic ester bonds add self-healing capabilities.

“It’s the first time we know of that any polymer has had all three of these functionalities together in one system,” says Michael Kessler, Director of the School of Mechanical and Materials Engineering at Washington State University. “I have seen other polymers with azobenzene in a liquid crystal network, but not in an epoxy system, and not in a system where there was a reversible crosslinking ester bond.”

In a remarkable video, Kessler and colleagues, including Yuzhan Li of WSU, show the LCN in action: A cube whose walls are thin films of the material unfolds into a flat, cross-shaped layer upon heating or irradiation with UV light, then reassembles itself into a cube again upon heating to a higher temperature.

To get to this result, the researchers synthesized an azobenzene-based epoxy monomer and polymerized it with an aliphatic dicarboxylic acid to form an LCN. A ring-opening/transesterification catalyst was added to promote formation of the LC phase and to activate the dynamic ester bonds created by the epoxy-acid reaction.

The LCN has two phase transition temperatures, which can be used as thermal triggers for the shape memory behavior: a glass transition temperature T_g at 51°C and a liquid crystal transition temperature T_lc at 103°C. The azobenzene chromophore strongly absorbs UV light and generates heat, providing a faster, photothermal trigger for the shape memory behavior. The dynamic ester bonds undergo transesterification between ester and hydroxyl groups at the topology freezing transition temperature T_v of 150°C. This fast breaking and reforming of ester bonds can be used to heal cracks in this polymer system, and even enable reprocessing of chopped up shreds of this material into a solid piece—quite unusual for a thermosetting resin.

To achieve the unfolding and refolding of the cube demonstrated in the video, the system has to be shape programmed first. There are three different shapes—a cube, a flattened cross-shape, and a collapsed cube—and two transitions (at T_g and T_lc) between these shapes, comprising three-way shape memory behavior. The cube is the initial and final stress-free shape, in this case, with two temporary shapes in the cycle. To change the permanent shape of the system, the researchers folded the flat-cross-shaped material into the cubic box configuration and heated it to 200°C, at which point the covalent bonds broke to form a stress-free cube. Upon cooling to 85°C, they manually opened and flattened the cube to form the first temporary flat-cross shape. The flat cross was then folded into a collapsed box (second temporary shape) at 85°C and cooled to room temperature in this form. When reheated, the collapsed box opened and reformed the flat cross shape at 85°C, then formed the permanent, stress-free cube shape again upon heating to 140°C.

Once programmed, the material undergoes this complex transition pattern by itself under UV irradiation or heat. Possible applications include actuators, drug delivery systems, or self-deploying antennas. The researchers plan to adjust the stoichiometry of this LCN and substitute different functional groups in the near future to optimize this system and explore the possible additional properties offered by other functional groups.

“This new report from Kessler and colleagues is an important contribution to the field through its combination of photoresponse, shape memory, and reconfigurability in one material,” says Patrick Mather, Dean of Engineering and Professor of Chemical Engineering at Bucknell University, who was not involved with this research. “The latter aspect, possible through bond exchange reaction (here, transesterification) is an emerging area in the field of smart polymers and I expect to see increasing activity in this area from Kessler and others.”

Read the article in ACS Applied Materials & Interfaces.